Various types of automotive lamps are partially covered with an opaque coating in order to prevent undesired stray radiation from the lamp which is not properly focused by the reflector in which it is to be mounted. It has previously been proposed to make such a cap of a mixture of carbonyl iron powder and cobalt (II, IV)--oxide--CO.sub.3 O.sub.4. The ratio of carbonyl iron powder to cobalt oxide can be in the order of between 1.5:1 to 4:1. In a usually used form, the ratio is between 5:3 to 11:6. The pigment is suspended in a vehicle, such as butanol, the bulb is dipped into the suspension of pigment and butanol, and after pulling out the bulb it is dried. The coating remaining on the bulb is then heated and sintered to a final temperature in the order of about 800.degree. C. This applies the coating to the lamp bulb. Prior to dipping, the end or cap portion of the bulb is heated to about 700.degree. C. in an oxidizing atmosphere to clean the cap of the bulb (see, for example British Patent Specification 1,334,329, German Disclosure Document DE-OS No. 21 36 012 and British Patent Specification 1,334,329 which contains similar disclosure).